Human life is distinguished from animal life due to its heavy responsibilities. Those who are cognizant of these responsibilities and who work in that spirit are called suras (godly persons), and those who are neglectful of these responsibilities or who have no information of them are called asuras (demons). Throughout the universe there are only these two types of human being. In the Ṛg Veda it is stated that the suras always aim at the lotus feet of the Supreme Lord Viṣṇu and act accordingly. Their ways are as illuminated as the path of the sun.
(Sri Isopanishad--purport).

There are swine, dogs, camels, asses, etc., whose economic necessities are just as important to them as ours are to us, but the economic problems of these animals are solved only under nasty and unpleasant conditions. The human being is given all facilities for a comfortable life by the laws of nature because the human form of life is more important and valuable than animal life. Why is man given a better life than that of the swine and other animals? Why is a highly placed government servant given better facilities than those of an ordinary clerk? The answer is that a highly placed officer has to discharge duties of a higher nature. Similarly, the duties human beings have to perform are higher than those of animals, who are always engaged in simply feeding their hungry stomachs. Yet the modern soul-killing civilization has only increased the problems of the hungry stomach. When we approach a polished animal in the form of a modern civilized man and ask him to take interest in self-realization, he will say that he simply wants to work to satisfy his stomach and that there is no need of self-realization for a hungry man.
(Sri Isopanishad--purport).

People are now addicted to eating animals, in spite of having an ample supply of grains, fruits and milk. There is no necessity for animal killing. This injunction is for everyone. When there is no alternative, one may kill an animal, but it should be offered in sacrifice. At any rate, when there is an ample food supply for humanity, persons who are desiring to make advancement in spiritual realization should not commit violence to animals. Real ahiṁsā means not checking anyone's progressive life. The animals are also making progress in their evolutionary life by transmigrating from one category of animal life to another. If a particular animal is killed, then his progress is checked. If an animal is staying in a particular body for so many days or so many years and is untimely killed, then he has to come back again in that form of life to complete the remaining days in order to be promoted to another species of life. So their progress should not be checked simply to satisfy one's palate. This is called ahiṁsā.
(Bhagavad-Gita----16:1-3----purport).

Religion includes four primary subjects, namely pious activities, economic development, satisfaction of the senses, and finally liberation from material bondage. Irreligious life is a barbarous condition. Indeed, human life begins when religion begins. Eating, sleeping, fearing, and mating are the four principles of animal life. These are common both to animals and to human beings. But religion is the extra function of the human being. Without religion, human life is no better than animal life. Therefore, in human societies there is some form of religion which aims at self-realization and which makes reference to man's eternal relationship with God.
(Srimad Bhagavatam----1:1:2----purport).

To effect the perfection of human life there is cooperation between men and demigods, sages, denizens of the Pitṛloka, devotees of the Lord and the scientific system of varṇa and āśrama orders of life. The distinction between human life and animal life therefore begins with the scientific system of varṇa and āśrama, guided by the experience of the sages in relation with the demigods, gradually rising to the summit of reestablishing our eternal relation with the Supreme Absolute Truth, the Personality of Godhead, Lord Śrī Kṛṣṇa. When God-made varṇāśrama-dharma, which is strictly meant for developing animal consciousness into human consciousness and human consciousness into godly consciousness, is broken by advancement of foolishness, the whole system of peaceful and progressive life is at once disturbed. In the age of Kali, the first attack of the venomous snake strikes against the God-made varṇāśrama-dharma, and thus a person properly qualified as a brāhmaṇa is called a śūdra, and a śūdra by qualification is passing as a brāhmaṇa, all on a false birthright claim. To become a brāhmaṇa by a birthright claim is not at all bona fide, although it may be a fulfillment of one of the conditions. But the real qualification of a brāhmaṇa is to control the mind and the senses, and to cultivate tolerance, simplicity, cleanliness, knowledge, truthfulness, devotion and faith in the Vedic wisdom.
(Srimad Bhagavatam----1:16:31----purport).

The Bhāgavatam says that the bellows of the blacksmith breathes very soundly, but that does not mean that the bellows has life. The materialist will argue that life in the tree and life in the man cannot be compared because the tree cannot enjoy life by eating palatable dishes or by enjoying sexual intercourse. In reply to this, the Bhāgavatam asks whether other animals like the dogs and hogs, living in the same village with human beings, do not eat and enjoy sexual life. The specific utterance of Śrīmad-Bhāgavatam in regard to "other animals" means that persons who are simply engaged in planning a better type of animal life consisting of eating, breathing and mating are also animals in the shape of human beings. A society of such polished animals cannot benefit suffering humanity, for an animal can easily harm another animal but rarely do good.
(Srimad Bhagavatam----2:3:18----purport).

One cannot be situated in an exalted position without having undertaken a regulative life of rules and regulations. An unrestricted life of sense gratification is animal life, and Lord Brahmā, in order to teach all concerned within the jurisdiction of his generations, taught the same principles of sense control for executing higher duties. He desired the welfare of all as servants of God, and anyone desiring the welfare of the members of his family and generations must conduct a moral, religious life. The highest life of moral principles is to become a devotee of the Lord because a pure devotee of the Lord has all the good qualities of the Lord. On the other hand, one who is not a devotee of the Lord, however qualified he may be in the mundane sense of the term, cannot be qualified with any good quality worthy of the name.
(Srimad Bhagavatam----2:9:40----purport).

The Supreme Personality of Godhead constantly lives with the individual soul as Paramātmā. The individual soul has awareness in accord with his material body, which he attains by virtue of prakṛti, or material nature. The material ingredients are activated by the force of time, and thus the three material modes of nature are manifested. According to his association with the three modes of nature, the living entity develops a particular type of body. In animal life, the material mode of ignorance is so prominent that there is very little chance of realizing the Paramātmā, who is also present within the heart of the animal; but in the human form of life, because of developed consciousness (cetanām), one can be transferred from ignorance and passion to goodness by the results of his activities (kriyā-phalatvena). A human being is therefore advised to associate with spiritually advanced personalities. The Vedas give the direction tad-vijñānārthaṁ sa gurum evābhigacchet: (MU 1.2.12) in order to reach the perfection of life or to understand the real constitutional position of the living entity, one must approach the spiritual master.
(Srimad Bhagavatam----4:21:35----purport).